There has constantly been a demand for miniaturization in an RF circuit portion of a cellular phone. In recent years, there has been a demand for various functions to be imparted to the cellular phone. To realize this, as many components as possible are preferably incorporated. On the other hand, since a size of the cellular phone is restricted, after all it is necessary to raise a mounting density in an apparatus. Therefore, a demand for reduction of an occupying area (mounting area) and height is strict particularly with respect to the RF circuit portion. Therefore, the occupying area and the height have been requested to be reduced even with respect to the components constituting the RF circuit portion. Moreover, each component has been requested to be constituted of a single part if possible, so that the portion can be manufactured at a low cost. Especially concerning a duplexer or transmission/reception switch which is one of the components constituting the RF circuit portion, sufficient power handling capability is required, and it is necessary not to cause destruction and characteristic deterioration with respect to power supply.
At present, for the duplexer, a ceramic filter or a surface acoustic wave (SAW) filter has been used. The ceramic filter can be monolithic and is inexpensive, but a loss of a resonator is large. To cover this, a dimension of about 23×7×5 mm is required, and the demand for the reduction of the mounting area or height cannot sufficiently be satisfied. On the other hand, the SAW filter is small in size, but has a problem in the power handling capability at a high frequency, and there is a possibility that the filter breaks down, when a surge is generated in an output power of transmission. Moreover, in order to improve characteristics, a circuit also using an electronic switch can also be constituted. However, in this case, the circuit constitution becomes complicated and expensive.
In consideration of these situations, in JP(A) 2001-24476, a duplexer is proposed in which separation between a transmission band and reception band is little, and use in applications such as CDMA-PCS devices requiring a higher power level is possible. The duplexer is constituted of three components including a band filter for transmission, band filter for reception, and 90-degrees phase shifter. A film bulk acoustic resonator (abbreviated as “FBAR”) constituting each filter has a very small size and high performance. However, the 90-degrees phase shifter is other than the film bulk acoustic resonator constituting each filter, and is constituted of passive components such as an inductor and capacitor. Therefore, for the duplexer described in the above JP(A) 2001-24476, a plurality of filters and 90-degrees phase shifter need to be mounted on one substrate, and it is difficult to constitute a monolithic structure and to sufficiently reduce the cost. It is also possible to form the 90-degrees phase shifter of the duplexer by a transmission line. However, in this case, a length of at least ten or more millimeters is necessary, a considerably large space is required as compared with the filter, and this constitution is unsuitable for miniaturization. That is, features of the film bulk acoustic resonator suitable for the miniaturization cannot sufficiently be utilized.
Moreover, when the duplexer is used in the above-described application, a pass band width of up to 4% of a central frequency is required in each filter (e.g., when the central frequency is 2 GHz, a pass band width of about 80 MHz is required). However, a piezoelectric material of the film bulk acoustic resonator for use in this application mainly contains AlN and ZnO. In this case, the required pass band width cannot easily be achieved. This is because the pass band width is limited by a piezoelectric coupling coefficient (K2) which is a material parameter. As one method for solving this problem, in JP(A) 2001-244704, it has been proposed that by connection of an external inductor element in series or in parallel to the film bulk acoustic resonator, a broad pass band width be obtained as if an effective K2 of the material increased. However, this method has a defect that the inductor is attached to the film bulk acoustic resonator from the outside and accordingly the whole size of the filter increases.
The present invention has been developed in consideration of the above-described situation, and an object thereof is to provide a duplexer in which a connection portion between a filter for transmission and a filter for reception can be structured to be small in size and which can be formed to be monolithic with the filter for transmission and the filter for reception and which is satisfactory in power handling capability and small in size and inexpensive.
Moreover, the present invention has been developed in consideration of the above-described situation, and an object thereof is to provide a duplexer in which a matching circuit for connecting a filter for transmission using a film bulk acoustic resonator to a filter for reception using the film bulk acoustic resonator or passive elements such as an inductor for obtaining a useful pass band width of each filter are formed to be monolithic with the filters for transmission and reception and which is accordingly small in size and satisfactory in power handling capability and easy to manufacture and can be reduced in cost.
Furthermore, an object of the present invention is to provide a filter for use in the above-described duplexer.